1. (Field of the Invention)
The present invention relates to a resin packaged semiconductor device and more particularly, to a resin packaged semiconductor device which is excellent in damp-proof and corrosion resistance.
2. (Description of the Prior Art)
A semiconductor device such as IC or LSI is generally packaged with various packaging material for the purpose of physical protection against the environment, electrical insulation or the like.
Recently, a resin packaging, which is advantageous in economical efficiency and mass-productivity, is frequently carried out as such packaging material and process. Particularly, transfer packaging with an epoxy resin is mainly employed.
Though packaging with an epoxy resin (hereinafter referred to simply as "resin") is advantageous in economical efficiency and mass-productivity, the package exhibits a poor resistance to moisture permeation, because it is composed of the epoxy resin. Therefore, some problems due to the permeation are brought about.
More precisely, moisture penetrates from the interface between the packaging epoxy resin and the lead frame or permeates through the packaging resin, while dissolving the molding resin component and impurities. Finally it reaches an aluminum metal on the semiconductor chip or an aluminum metallization to form a corrosive liquid film on these parts, thus causing corrosion.
Various methods for controlling such corrosion have been proposed. One of them is a method which comprises forming a protective film on the surface of aluminum metal or aluminum metallization on semiconductor device to thereby control the corrosion.
For example, U.S. Pat. No. 3735484 discloses a method which comprises immersing the part of a semiconductor device to be treated in an aqueous phosphate solution containing anhydrous chromic acid, ammonium fluoride and phosphoric acid to treat the aluminum surface with the phosphate and to form a passivated film on the aluminum surface.
And also Japanese Patent Laid-Open No. 23979/1975 discloses a method which comprises bonding aluminum metal or aluminum metallization to a lead wire and immersing the resulting device in an aqueous solution of phosphoric acid and chromic acid to form a passivated film on the aluminum surface.
According to these methods, a passivated film is formed on the surface of aluminum metal or aluminum metallization to satisfactorily control the corrosion. However, a semiconductor device not always has an aluminum or aluminum alloy surface. That is, a lead frame has sometimes a surface composed of copper, 42N.sub.i -F.sub.e alloy or silver plating, while a lead wire for wire bonding is sometimes made of gold or copper. In such a case, the treatment according to the above method case does not give any passivated film, so that potential difference is generated in this part to form a corrosion cell. Therefore, the corrosion proceeds, which adversely affects the reliability.
Further, as a method of forming a protective film, Japanese Patent Laid-Open No. 116634/1981 discloses a process which comprises exposing a semiconductor device after wire bonding to an atmosphere of high-temperature water vapor to form an oxide layer on the aluminum surface.
And also Japanese Patent Laid-Open No. 50687/1977 discloses a similar method which comprises immersing a semiconductor device in a hot water of 80.degree. to 250.degree. C. for 5 to 100 minutes to form a hydrated oxide layer on the aluminum surface.
These methods are expected to give corrosion-decreasing effect due to the oxide or hydrated oxide layer formed on the aluminum surface. However, as described above, the semiconductor-constituting metal is not always aluminum alone, but also copper, silver, 42N.sub.i -F.sub.e alloy and the like.
Therefore, when such a semiconductor containing a metal other than aluminum is treated according to the above method which comprises exposing a semiconductor to high-temperature water or water vapor, though the semiconductor-constituting materials other than aluminum or its alloy corrode to result in decreased corrosion of aluminum or its alloy, the corrosion of other metals tends to be promoted.
Further, the above methods which use water as an aqueous solution or water vapor are not preferred in terms of control of the corrosion of semiconductor-constituting metals.
Furthermore, for example, Japanese Patent Laid-Open Nos. 115783/1974 and 140476/1976 disclose methods characterized by plating an aluminum metal or aluminum metallization. These methods comprise plating an aluminum metal or aluminum metallization with an anticorrosive metal to thereby protect the aluminum metal or aluminum metallization from a corrosive environment.
However, both electric and chemical plating methods use an aqueous solution, the pH of which is generally on an acid side. Therefore, this plating method has disadvantages in that the semiconductor-constituting metal is somewhat dissolved and that the impurities and anions contained in a plating solution adhere to the aluminum metal or aluminum metallization, so that the aluminum metal or aluminum metallization must be sufficiently washed and dried after plating, thus increasing the number of steps and treatment time.